Method for temperature compensation of an image sensor sensitivity

ABSTRACT

A method for temperature compensation of sensitivity of an image including photosensitive spots each with a photodiode connected to read circuits. The photosensitive spots are divided into detecting photosensitive spots, detecting an image when exposed to information carrying the image and sensitive to this information, and into blind spots protected from the information. When the photosensitive spots are taken to a reference temperature, an average leakage current in the photodiodes of the blind photosensitive spots is calculated and a first average is generated from signals from the blind photosensitive spots during a read operation. When the photosensitive spots are taken to an ambient temperature to be determined, another average is generated from signals from the blind photosensitive spots during another read operation. The ambient temperature is calculated from the average leakage current and from the distance between the two averages. A gain image or a quasi gain image matched to the ambient temperature is generated. An image recorded at the ambient temperature with the gain image or the quasi gain image is then corrected. Such a method may find application to radiological image detectors in particular.

The present invention relates to solid-state image detectors and itspurpose is to eliminate the variations in their sensitivity, especiallythose due to temperature variations.

In these image detectors, the acquisition of an image takes place withthe aid of several photosensitive spots each formed from a photodiodeand a switch. The photosensitive spots are produced with the aid oftechniques for the thin-film deposition of semiconductor materials suchas hydrogenated amorphous silicon (aSiH). These photosensitive points,arranged in the form of a matrix or linear array, make it possible todetect images contained within visible or near-visible radiation. Thesignals that are produced are then digitized so as to be able to bestored and processed easily.

These arrangements of photosensitive spots find one particularlyadvantageous application in the medical field or the field of industrialinspection, in which they detect radiological images. All that isrequired is to cover them with a scintillator and to expose the latterto X-radiation carrying a radiological image. The scintillator convertsthe incident X-radiation into radiation in the band of wavelengths towhich the photosensitive spots are sensitive.

There are now large photosensitive matrices which may have severalmillion photosensitive spots. FIG. 1 shows a matrix image detector ofthe known type. It has only nine photosensitive spots in order notunnecessarily clutter up the figure. Each photosensitive spot P1 to P9is formed from a photodiode Dp and an element having a switch functionDc represented in the form of a switching diode. It would have beenpossible to choose a transistor as the element having a switchingfunction. The photodiode Dp and the switching diode Dc are connectedtogether in a head-to-tail arrangement.

Each photosensitive spot P1 to P9 is connected between a row conductorY1 to Y3 and a column conductor X1 to X3. The row conductors Y1 to Y3are connected to an addressing device 3 known as a driver. There may beseveral drivers 3 if the matrix is of large size. The addressing device3 generally comprises shift registers, switching circuits and clockcircuits. The addressing device 3 raises the row conductors Y1 to Y3 tovoltages which either isolate the photosensitive spots P1 to P3connected to the same row conductor Y1 from the rest of the matrix orturn them on. The addressing device 3 allows the row conductors Y1 to Y3to be addressed sequentially.

The column conductors X1 to X3 are connected to a read device CL.

During an image record phase, during which the photosensitive spots P1to P9 are exposed to information to be picked up and are in a receivingstate, that is to say their reverse-biased photosensitive diodes Dp andswitching diodes Dc each constitute a capacitor, there is a build up ofcharges at the junction point A between the two diodes Dp, Dc. Theamount of charge is approximately proportional to the intensity of thesignal received, whether this is very intense illumination, providedthat the photosensitive diodes remain in the linear detection range, ordarkness. There then follows a read phase, during which a read pulse issequentially applied to the row conductors Y1 to Y3, which read pulseturns on the photodiodes Dp and makes it possible for the chargesaccumulated in the column conductors X1 to X3 to drain away to the readdevice CL and for them to be integrated.

A read device CL will now be explained in greater detail. It consists ofas many read circuits 5 as there are column conductors X1 to X3 andthese read circuits are of the charge-integrating circuit type. Eachphotosensitive spot is connected to a read circuit 5. Eachcharge-integrating circuit is formed by an operational amplifier G1 toG3 mounted as an integrator by means of a read capacitor C1 to C3. Eachcapacitor C1 to C3 is mounted between the negative input of theoperational amplifier G1 to G3 and its output S1 to S3. Each columnconductor X1 to X3 is connected to the negative input of an operationalamplifier G1 to G3. The positive input of each of the operationalamplifiers G1 to G3 is taken to a constant input reference voltage VR,which sets this reference voltage on each column conductor X1 to X3.Each operational amplifier G1 to G3 comprises a resetting switch I1 toI3 mounted in parallel with the capacitor C1 to C3.

The outputs S1 to S3 of the integrating circuits are connected to amultiplexing device 6 which delivers, as a series, signals correspondingto the charges which were integrated by the charge-integrating circuits.In the read phase, these signals correspond to the charges accumulatedover an integration time by all the photosensitive spots of the samerow. The signals delivered by the multiplexing device 6 are thendigitized in at least one analog-digital converter 7, the digitizedsignals output by the analog-digital converter 7 translating the contentof the image to be detected. These digitized signals are sent to amanagement system 8 which can store, process and display them.

It has turned out that the sensitivity of such detectors varies, whichresults both in local and overall variations in the brightness of theimage detected. There are several causes of the variations insensitivity. Firstly, there is a spatial variation and secondly athermal variation. This means, on the one hand, that two photosensitivespots of the detector cannot give the same response when they areexposed to precisely the same luminous flux and, on the other hand, thata photosensitive spot exposed to the same luminous flux does not givethe same response at 25° C. as it does at 35° C. These discrepancies arepartly due to the semiconductor components constituting thephotosensitive spots, which components do not all come from the samemanufacturing batch, and partly to the scintillator material used inradiology. This results in images with nonuniform areas which should notbe there and which become increasingly pale as the temperatureincreases.

Although it is known how to overcome the spatial variation insensitivity by making a correction to the image with a so-called gainimage, it is not possible to use the gain image to overcome thermallyinduced variations in sensitivity.

The gain image is an image taken with a calibrated uniform illuminationin the absence of a subject or object to be examined. This gain imageallows the spatial variations in sensitivity to be properly corrected,since with a uniform illumination the image should be uniform. This gainimage is produced with a very low frequency, of the order of one year.The signals delivered by the photosensitive spots when the gain image isbeing read are stored in the management device 8 and then serve tocorrect, for spatial inhomogeneity in the sensitivity, any useful image.

This method cannot be used to overcome thermally induced variations insensitivity: this would require producing gain images in synchronismwith the temperature variations, which would significantly increase thefrequency at which gain images are recorded. This is not compatible withthe manner in which operators use such image detectors.

The present invention proposes the use of a gain image or a quasi gainimage matched to the ambient temperature in order to obviate variationsin the sensitivity of the image detector, especially thermally inducedvariations, but this gain image is not simply recorded just beforemaking the correction, in order to be matched to the ambienttemperature, but it is generated from a calculation resulting in thedetermination of the ambient temperature.

To achieve this, the present invention provides a method for temperaturecompensation of the sensitivity of an image comprising photosensitivespots, each with a photodiode, these being connected to read circuits,characterized in that the photosensitive spots are divided intodetecting photosensitive spots, capable of detecting an image when theyare exposed to information carrying the image and are sensitive to thisinformation, and into blind spots protected from the information, and inthat it consists:

when the photosensitive spots are taken to a reference temperature, incalculating an average leakage current in the photodiodes of the blindphotosensitive spots and in generating an average from the signalsdelivered by the blind photosensitive spots during a read operation;

when the photosensitive spots are taken to an ambient temperature to bedetermined, in generating another average from the signal delivered bythe blind photosensitive spots during another read operation;

in calculating the ambient temperature from the average leakage currentand from the distance between the two averages;

in generating a gain image or a quasi gain image matched to the ambienttemperature; and

in correcting an image recorded at the ambient temperature with the gainimage or the quasi gain image.

Preferably, the signals delivered for generating the first average andfor generating the average at the ambient temperature to be determinedcorrespond to charges integrated over a first integration timeapproximately equal to the nominal integration time of the imagedetector.

To calculate the average leakage current at the average referencetemperature, it is possible to generate, at the reference temperature, apair of averages from the signals delivered by the blind photosensitivespots over two different integration times and to make a calibrationusing the pair of averages.

One of the averages of the pair is advantageously the first average. Theother average of the pair corresponds to charges integrated over anintegration time longer than the nominal integration time of the imagedetector.

The gain image matched to the determined ambient temperature may begenerated from a series of gain images stored beforehand in a memorydevice, each of them being recorded at a different particulartemperature, all of these particular temperatures forming a range oftemperatures at which the image detector is likely to operate.

The quasi gain image matched to the determined ambient temperature maybe generated, advantageously, from a base gain image recorded at a basetemperature and corrected with the aid of a coefficient of variation ofthe base gain image as a function of temperature and taking into accountthe difference between the determined ambient temperature and the basetemperature.

The base temperature may be the reference temperature. Advantageously,the averages may be generated from black images.

The present invention also relates to an image detector for implementingthe compensation method, comprising photosensitive spots each with aphotodiode, these photosensitive spots being connected to read circuits.These photosensitive spots are divided into detecting photosensitivespots, capable of detecting an image when they are exposed toinformation carrying the image, and into blind spots protected from theinformation. The detector comprises means for calculating the averagesfrom the signals delivered by the blind photosensitive spots, means forcalculating the average leakage current in the diodes of the blindphotosensitive spots, means for calculating the ambient temperature fromthe average leakage current and from the distance between averages,means for generating the gain image or the quasi gain image from thecalculated ambient temperature and means for correcting an imagerecorded at the ambient temperature with the gain image or the quasigain image.

The means for calculating the average leakage current receive theaverages of the signals delivered by the blind photosensitive spots indigital form. The means for generating the gain image may comprise amemory device containing one or more gain images, each corresponding toone temperature.

The means for generating the quasi gain image may comprise a memorydevice containing a base gain image recorded at a base temperature and acoefficient of variation of the base image with temperature.

It is preferable for the blind photosensitive spots to be connected tooutermost portions of conductors to which the detecting photosensitivespots are connected. The blind photosensitive spots are covered with amaterial opaque to the information received by the detectingphotosensitive spots, this material being in particular black paint.

The detecting photosensitive spots are covered with a scintillatormaterial which converts X-radiation into radiation to which they aresensitive, the blind photosensitive spots being covered with anX-ray-opaque material, such as lead.

The material opaque to the information lies between the X-ray-opaquematerial and the blind photosensitive spots.

Further features and advantages of the invention will become apparent onreading the description which follows, illustrated by the figures, inwhich:

FIG. 1, already described, shows an example of a known image detector;and

FIG. 2 shows an example of an image detector according to the invention,capable of operating with the method according to the invention.

In the manner indicated in FIG. 1, the photosensitive spots O1 to O6 andR1 to R9 are shown with a photodiode Dp and an element having a switchfunction Dc shown in the form of a switching diode. This switching diodecould be replaced by a transistor. The photodiode Dp and the switchingdiode Dc are connected together in a head-to-tail arrangement. Eachphotosensitive spot is connected between a row conductor Y1 to Y3 and acolumn conductor W1, W2 and Z1 to Z3. The photosensitive spots O1 to O6and R1 to R9 are arranged in a matrix of rows and columns, but theycould be arranged in a linear array. Compared with the example in FIG.1, the image detector shown has more photosensitive spots and morecolumn conductors, but the same number of row conductors. The rowconductors are connected to an addressing device 3 similar to thatdescribed in FIG. 1.

According to one feature of the invention, the photosensitive spots aredivided into two groups—detecting photosensitive spots R1 to R9 which,when they are exposed to information carrying an image and are sensitiveto this image, are capable of detecting the image, and blindphotosensitive spots O1 to O6 used for compensation. These blindphotosensitive spots O1 to O6 are masked from the information carryingan image to be detected. During detection of an image, whether this isthe image of an object or of a patient, or even a black (unilluminated)or gain image, the blind photosensitive spots receive nothing. Theseblind photosensitive spots O1 to O6 will be read in the same way as thedetecting photosensitive spots R1 to R9.

The blind photosensitive spots O1 to O6 are connected to outermostportions 20 of the row conductors Y1 to Y3. In the example described,they are located at the start of a row, but they could be located at theend of a row.

The number of blind photosensitive spots is not critical—about 10 perrow seems reasonable if a row numbers about 2,000 detectingphotosensitive spots. These photosensitive spots O1 to O6, R1 to R9 areimplanted in an insulating substrate with the reference 21.

To mask the blind photosensitive spots O1 to O6 from the information towhich the detecting photosensitive spots are exposed, they are coveredwith a material PN opaque to the information received by the detectingphotosensitive spots—black paint for example is very suitable.

In the configuration in which the image detector according to theinvention is used in a radiological application, the detectingphotosensitive spots R1 to R9 are covered with a scintillator materialSC which coverts X-radiation into radiation in the band of wavelengthsat which the detecting photosensitive spots R1 to R9 are sensitive. Asregards the blind photosensitive spots O1 to O6, these are not coveredwith the scintillator material SC but with an X-ray-opaque material PB,for example a layer of lead. In this configuration, the material PNopaque to the information received by the detecting photosensitive spotsis optional, but if one is used it is placed between the blindphotosensitive spots O1 to O6 and the X-ray-opaque material PB.

The entire surface of the image detector on that side facing theX-radiation is covered with a protective material PP based for exampleon carbon fibers. In FIG. 2, these materials are shown only partly.

As in the example in FIG. 1, the read device CL has as many readcircuits 30 a, 30 b as there are column conductors W1, W2 and Z1 to Z3and each of these read circuits is of the charge-integrating circuittype with an operational amplifier Ga, Gb mounted as an integrator withthe aid of a read capacitor 31 a, 31 b and a resetting switch Ia, Ibmounted in parallel with the read capacitor 31 a, 31 b. As in FIG. 1,the outputs 32 a, 32 b of the integrating circuits Ga, Gb are connectedto a multiplexing device 60 which delivers, as a series, signalscorresponding to the charges which were integrated by thecharge-integrating circuits. The signals delivered by the multiplexingdevice 60 are then digitized in at least one analog-digital converter(ADc) 70. The signals output by the read circuits 30 b connected to thedetecting photosensitive spots R1 to R9 translate the image to bedetected, while the other signals output by the read circuit 30 aconnected to the blind photosensitive spots O1 to O6 serve forcompensation.

The digitized signals are then transmitted to a management system 80which can store, process and display them.

According to the method of the invention, when the photosensitive spotsare taken to a reference temperature θref, an average leakage currentIθref in the photodiodes of the blind photosensitive spots is calculatedand a first average COD1 is generated from the signals delivered by theblind photosensitive spots O1 to O6 during a read operation. Thereference temperature may be measured by means of a thermometer.

When the photosensitive spots are taken to an ambient temperature θ tobe determined, another average COD1′ is generated from the signalsdelivered by the blind photosensitive spots O1 to O6 during another readoperation.

The two averages COD1, COD1′ are used in digital form and it ispreferable for them to be produced from the signals delivered by theblind photosensitive spots already converted by the analog-digitalconverter 70.

It may even be envisioned that it is the analog-digital converter 70which delivers the averages. However, it is possible to produce theaverages in analog form and to carry out the conversion subsequently. Inthe non-limiting example in FIG. 2, the means 700 for generating theaverages are shown schematically by dashed lines and include the analogdigital converter 70.

The signals used to generate the first average COD1 correspond to thecharges stored by the blind photosensitive spots with a firstintegration time t1. This first integration time is preferably more orless the nominal integration time of the detector, that is to say theintegration time corresponding to a standard use of the detector. Forexample, t1 may be given a value of between 0.5 and 5 seconds.

Likewise, the signals used to generate the other average COD1′, at theambient temperature to be determined, correspond to the charges storedby the blind photosensitive spots with an integration time t1′ and it ispreferable for this integration time t′1 to be approximately equal tothe nominal integration time of the detector and therefore approximatelyequal to t1.

The ambient temperature θ is calculated from the difference between theaverages COD1′, COD1 and from the average leakage current Iθref at thereference temperature.

Next, a gain image GI or a quasi gain image QGI matched to thedetermined ambient temperature θ is generated in order to correct adetected image, although the photosensitive spots are still at thedetermined ambient temperature θ.

An example of how these various steps are carried out will now bepresented. The leakage current in a photodiode varies exponentially withtemperature. This leakage current is given by the following formula:

Iθ=Iθref×10^((θ−θref)/θ).

The current IP from a photosensitive spot is given by:${Ip} = \frac{{COD} \times {FSR} \times C_{read}}{2^{n}{Gt}}$

with:

COD: the code, expressed in binary (LSB), delivered by theanalog-digital converter 70 during an operation to read the chargesstored by this photosensitive spot after an integration time of t. Theintegration time t corresponds to the time elapsed between twosuccessive trainings of the charges accumulated at point A of thephotosensitive spot. COD can take values from 0 to 2¹⁴ if the converterpossesses 14 bits;

FSR: the encoding voltage range of the analog-digital converter. Thisrange may have a value of 4 volts for example;

n is the resolution of the analog-digital converter. This resolution maybe 14 bits for example;

C_(read) represents the equivalent read capacitance at the output of theread circuit CL; and

G: the voltage gain separating the output of the read circuit from theinput of the analog-digital converter.

During an operation to read a blind photosensitive spot O1 to O6 (whichtherefore has not been exposed to illumination), the charges accumulatedby this photosensitive spot do not correspond exactly to the leakagecurrent of the photodiode. These charges also include drive chargescreated in the photosensitive spot from the pulses which the addressingdevice 3 receives and from the charges coming from the photosensitivespots connected to the same column conductor as that which is read.

To obviate these additional charges, a calibration operation is carriedout to calculate the leakage current. When the photosensitive spots aretaken to the reference temperature θ ref, a pair of averages COD1, COD2is generated from the signals delivered by the blind photosensitivespots with two different integration times.

It has been assumed below, for the purpose of nonlimiting simplificationthat one of the averages of the pair, COD1, corresponds to thatgenerated at the reference temperature θ ref, but for the purpose ofcalculating the temperature θ to be determined. The other average, COD2,of the pair is generated from the signals delivered by the blindphotosensitive spots O1 to O6 during a second read operation at thereference temperature θref.

The integration time t1 relating to the first average COD1 of the pairis approximately the nominal integration time of the detector. Theintegration time t2 relating to the second average COD2 of the pair islonger than the first time t1. It may be chosen for example to be 2 to10 times longer than the time t1. This time t2 may have a value from 1to 20 seconds.

It is preferable that the read operations for calculating the averageleakage current and for generating the averages be carried out fromblack images obtained when the photosensitive spots are not exposed toany illumination. In a radiological application, this makes it possibleto dispense with X-radiation. However, the use of useful images could beenvisioned.

With these two averages COD1, COD2, it is then easy to calculate theaverage leakage current Iθ ref at the reference temperature θref fromthe formula:${I\quad \theta \quad {ref}} = {\frac{\left( {{COD2} - {COD1}} \right) \times {SFR} \times C_{read}}{2^{n}{G\left( {{t2} - {t1}} \right)}}.}$

The ambient temperature θ to be determined may then be calculated fromthe formula:$\theta = {\theta \quad {{ref}\left\lbrack {1 + {\log \left( {1 - \left\lbrack \frac{\left( {{COD1}^{\prime} - {COD1}} \right) \times {FSR} \times C_{read}}{2^{n} \times G \times I\quad \theta \quad {ref} \times {t1}^{\prime}} \right\rbrack} \right)}} \right\rbrack}}$

When the ambient temperature θ has been calculated, all that is requiredis to generate a gain image GI or a quasi gain image QGI matched to thisambient temperature θ, and this gain image GI or this quasi gain imageQGI will be used for digitally correcting, in terms of sensitivity, auseful image at the temperature θ.

The gain image GI matched to the temperature may be generated from aseries of gain images GI₁ . . . GI_(n) stored beforehand in a memorydevice 100, each of them being associated with one temperature. Theimage detector is subjected beforehand to a series of temperatures atwhich there is a risk of it having to operate, and for each of them, again image GI₁ . . . GI_(n), stored in the memory device 100 so as toconstitute a library of gain images GI₁ . . . GI_(n) is recorded.

If a less precise correction is acceptable, all that is required is torecord a base gain image bGI at a base temperature and determine acoefficient K of variation of the base gain image as a function oftemperature. The base gain image bGI is stored in the memory device 100.This variant requires appreciably less memory capacity. The quasi gainimage QGI is obtained by applying the coefficient K to the base gainimage bGI using the difference between the base temperature and thecalculated ambient temperature. The base temperature may also be thereference temperature θref.

FIG. 2 shows the two variants illustrated in the same memory device, butthis is not limiting. Measurements performed show that the coefficient Kis around −0.5%/° C.

All the functions that have just been described may be produced by themanagement device 80 itself, or by any processing unit 800 placedbetween the analog-to-digital converter 70 and the management device 80,which then allows the corrected images to be used. FIG. 2 shows indetail the configuration with a processing unit 800 which includes themeans 81 for calculating the average leakage current, the means 82 forcalculating the ambient temperature θ to be determined, the means 83 forgenerating the gain image GI or the quasi gain image QGI which cooperatewith the memory device 100 and the correction means 84. These correctionmeans 84 receive the signals delivered by the photosensitive spotsduring an operation to read a useful image to be corrected. Thesesignals are delivered by the analog-digital converter 70.

What is claimed is:
 1. A method for temperature compensation ofsensitivity of an image including photosensitive spots each with aphotodiode, the photodiodes being connected to read circuits, whereinthe photosensitive spots are divided into detecting photosensitivespots, configured to detect an image when they are exposed toinformation carrying the image and are sensitive to this information,and into blind spots protected from the information, comprising: whenthe photosensitive spots are taken to a reference temperature,calculating a first average leakage current in the photodiodes of theblind photosensitive spots and generating a first average from signalsdelivered by the blind photosensitive spots during a first readoperation; when the photosensitive spots are taken to an ambienttemperature to be determined, generating a second average from signalsdelivered by the blind photosensitive spots during a second readoperation; calculating the ambient temperature from the average leakagecurrent and from a difference between the first and second averages;generating a gain image or a quasi gain image matched to the ambienttemperature; and correcting an image recorded at the ambient temperaturewith the gain image or the quasi gain image.
 2. The compensation methodas claimed in claim 1, wherein the signals delivered for generating thefirst average and for generating the second average at the ambienttemperature to be determined correspond to charges integrated over anintegration time approximately equal to a nominal integration time ofthe image detector.
 3. The compensation method as claimed in claim 1,further comprising, at the reference temperature, to calculate theaverage leakage current, generating a pair of averages from the signalsdelivered by the blind photosensitive spots over two differentintegration times and making a calibration using the pair of averages.4. The compensation method as claimed in claim 3, wherein one average ofthe pair of averages is the first average.
 5. The compensation method asclaimed in claim 4, wherein the other average of the pair of averagescorresponds to charges integrated over an integration time longer than anominal integration time of the image detector.
 6. The compensationmethod as claimed in claim 1, further comprising generating the gainimage matched to the determined temperature from a series of gain imagespreviously stored in a memory device, each of the series of gain imagesrecorded at a different particular temperature, all of the particulartemperatures forming a range of temperatures at which the image detectoris likely to operate.
 7. The compensation method as claimed in claim 1,further comprising generating the quasi gain image matched to thedetermined ambient temperature, from a base gain image recorded at abase temperature, corrected with aid of a coefficient of variation ofthe base gain image with temperature, taking into account a differencebetween the calculated ambient temperature and the base temperature. 8.The compensation method as claimed in claim 1, further comprisingcarrying out the read operations for generating the first and secondaverages on a black image.
 9. An image detector for implementing themethod as claimed in claim 1, comprising: the photosensitive spots eachwith the photodiode, the photosensitive spots being connected to theread circuits, wherein the photosensitive spots are divided into thedetecting photosensitive spots, configured to detect an image whenexposed to information carrying the image, and into the blind spotsprotected from the information; means for calculating the first andsecond averages from the signals delivered by the blind photosensitivespots; means for calculating the average leakage current in thephotodiodes of the blind photosensitive spots; means for calculating theambient temperature from the average leakage current and from thedifference between averages; means for generating the gain image or thequasi gain image from the calculated ambient temperature; and means forcorrecting an image recorded at the ambient temperature with the gainimage or the quasi gain image.
 10. The image detector as claimed inclaim 9, wherein the means for calculating the average leakage currentreceives the first and second averages in digital form.
 11. The imagedetector as claimed in claim 9, wherein the means for generating thegain image comprises a memory device containing one or more gain imageseach corresponding to one temperature.
 12. The image detector as claimedin claim 9, wherein the means for generating the quasi gain imagecomprises a memory device containing a base gain image recorded at abase temperature and a coefficient of variation of the base image withtemperature.
 13. The image detector as claimed in claim 9, wherein theblind photosensitive spots are connected to outermost portions of theconductors to which the detecting photosensitive spots are connected.14. The image detector as claimed in claim 9, wherein the blindphotosensitive spots are covered with a material opaque to theinformation received by the detecting photosensitive spots.
 15. Theimage detector as claimed in claim 14, wherein the material comprisesblack paint.
 16. The image detector as claimed in claim 14, wherein thedetecting photosensitive spots are covered with a scintillator materialthat converts X-radiation into radiation to which they are sensitive,the blind photosensitive spots being covered with an X-ray-opaquematerial.
 17. The detector as claimed in claim 16, wherein theX-ray-opaque material comprises lead.
 18. The detector as claimed inclaim 16, wherein the material opaque to the information lies betweenthe X-ray-opaque material and the blind photosensitive spots.